1. Field of the Invention
The present invention relates to a metal halide lamp with no mercury sealed therein and a lighting device using this lamp.
2. Description of the Related Art
In high-pressure discharge lamps, a pair of electrodes are sealed in a discharge space defined in a light-transmitting, airtight container made of refractory materials, and a discharge medium using a metal vapor as a main component is sealed in the container. Such electrodes generally have a structure wherein: Their proximal ends are welded to respective metal leaves airtightly buried in a pair of slim sealing portions formed integral with the opposite ends of the airtight container. Their intermediate portions are loosely supported by the respective sealing portions. Further, their distal ends, i.e., electrode main portions, protrude into the discharge space.
High-pressure discharge lamps are used for various purposes. Among them, compact high-pressure discharge lamps of a high output used in, for example, vehicle headlights are characterized in that their airtight container has a small internal volume, the pressure of the discharge medium is high during lighting, and the operating temperature is high. Therefore, the influence of impurities discharged from their structural components mounted on or sealed in the airtight container upon the long-term brightness or life of the lamps is relatively high.
Furthermore, in the high-pressure discharge lamps for vehicle headlights, the luminous power. 1.5 output immediately after ignition is lower than a predetermined value. To compensate for this, power several times higher than in a stable state is supplied at the start of lighting. More specifically, immediately after ignition, a lamp current several times larger than in the stable state is produced between the electrodes, thereby accelerating increase in luminous power to promptly activate the high-pressure lamp. At the same time, the lamp is controlled to be promptly stabilized.
On the other hand, high-pressure discharge lamps with an internal volume of 0.1 cc or less, which are used as metal halide lamps for vehicle headlights or spot lights, generally have a structure in which a rare gas, halides of light-emitting metals and mercury are sealed in a light emission tube with a pair of opposing electrodes. These high-pressure discharge lamps exhibit a relatively high efficiency and a high color-rendering characteristic. Therefore, they are widely used. However, at the present stage at which environmental problems have become serious, it has become significantly important also in the field of lighting devices to reduce or stop the use of mercury whose environmental impact is high. To this end, various proposals have been made for eliminating mercury from metal halide lamps. For example, Jpn. Pat. Appln. KOKAI Publication No. 11-238488 discloses a technique for adding, instead of mercury, a material having a high vapor pressure, such as ZnI2, to a light-emitting halide material, such as ScI3—NaI, thereby acquiring the same electric characteristic and light emission characteristic as those acquired from a mercury-containing lamp.
However, metal halide lamps without mercury cannot provide the advantage of thickening a discharge arc, obtained when the light emitted from mercury atoms is absorbed by the atoms themselves. Therefore, the resultant discharge arc is inevitably thin. The thickness (width) of a discharge arc influences the design of optical systems, and hence it is stipulated in regulations (e.g., EU Regulation No. 99 and Japan Electric Lamp Manufacturers Association Regulation JEL 215 “Vehicle Headlight HID Light Source”). If the arc is thin, it departs from the regulation.
The inventor of the present invention has found that when chromium (Cr), an impurity, exists in the airtight container, it emits light and thins the discharge arc, and that the discharge arc can have a thickness satisfying the regulation if the emission spectrum of chromium is kept at a predetermined value or less. The present invention has been developed based on this finding.